A quality control device is known from EP 0 527 453 B1. Quality control devices of this type are employed in connection with printing presses for use in taking images of the imprinted sheets and for detecting deviations of those images from a desired print quality. This is accomplished by comparing these sheets with a desired printed image. It is desirable to be able to intervene correctively in the printing process when required.
For this purpose, the above-described device has a suction box with a suction surface, over which suction surface the imprinted sheets are pulled with the aid of sheet grippers. Because of the frictional contact of the sheets with the suction surface of the suction box, it is assured that the sheets are always tightly stretched and that they lie on a well-defined level. It is thus possible to obtain a true image of the sheet with the aid of a camera facing the suction surface.
If such a known device is employed for the quality control of sheets, which have been imprinted on both sides, there is a danger that ink will be smeared over the surface of the sheet which is not controlled and which faces the suction box.
A device for quality control which addresses this problem is described in DE 44 36 583 A1. This prior device is comprised of a drum that is charged with a partial vacuum and a line camera which is facing the jacket of the drum. A gripper pulls the sheets to be controlled through the space between the line camera and the drum, where the sheets rest on the drum as a result of the partial vacuum. The path of the sheets through the quality control device is approximately straight. However, the suction effect provided by the drum causes each of the sheets to be controlled to adhere on a section of the drum in the shape of a segment of a circle. The extension of this section cannot be exactly controlled. It is a function of the stiffness of the sheets to be controlled and of the suction force exerted by the drum. It does not remain constant during the movement of a sheet over the drum. This has the result that, even if the speed of the gripper is exactly constant, the speed of that part of the surface of the sheet to be controlled which touches the drum and which lies within the field of view of the line camera, is subject to fluctuations in an uncontrolled manner. Since these fluctuations are not being taken into consideration in the course of the image capture by the line camera, non-reproducible variations of the results of the image-taking occur. If these fluctuations are not to result in the removal of some good sheets, the comparison of the sheets being printed, with the desired print image, must be “tolerant”. This results in the danger that errors actually present in the print image are overlooked.
A suction conveyor for use in a sheet-fed printing press is known from EP 0 798 251 A2, which suction conveyor is comprised of a suction box with a circulating driven belt. This suction box is employed for conveying the sheets at the start or the end of processing in the printing press at locations where sheet grippers, which are used for conducting the sheets between the various stations of the printing process have either not yet gripped them or have already released them.
However, in this device, the suction conveyor is used as the sole conveying mechanism for the sheets. In contrast, in the present invention, the suction belt which is employed is exclusively used for stabilizing, but not for conveying the sheets, which are conveyed by a separate chain gripper system, for the purpose of quality control.
DE 42 39 561 A1 discloses a sheet conveying arrangement in which the start of the sheet is transported by a chain gripper and the imprinted underside of the sheet simultaneously rests on a moving suction belt. Nothing can be found in this document regarding the type of a drive mechanism used and the speed of the suction belt.